The underlying rendering engine (Gecko, FirefoxOS "Boots to Gecko" hence the codename is "B2G") is the same - all apps (including the Browser app on the phone) are written in HTML5/JS/CSS: https://wiki.mozilla.org/B2G

B2G is different than Desktop Firefox in some ways, for instance it uses multiple processes (this is being worked on for Desktop, it's already in use for plugins for instance) and supports some new APIs to access the hardware on the phone that you'd expect: https://wiki.mozilla.org/WebAPI

Gecko is shared. Everything else is different.